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So we now get started with the basics

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of Kubernetes and of interacting

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with our Kubernetes cluster.

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We learned how to create a deployment

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with the create deployment command.

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We of course, also learned how to update the image

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of deployment with the set image command.

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And we saw a couple of other commands

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like the expose command as well.

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But there's one potential problem with all these commands.

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For one, you of course have to memorize them.

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But that's not too difficult,

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since you'll use the same set of commands all the time.

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But you also have to repeat them all the time.

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It's a little bit like earlier in the course,

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where we always execute a docker run.

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And if we had a project with multiple containers in it,

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we had to run docker run quite a lot.

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That was when we learned about docker-compose,

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which all of a sudden allowed us to write down

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how containers should be built and started

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in a compose yaml file, and docker-compose up

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which then pick up that file and apply these instructions

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and build and start multiple containers at once.

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And therefore docker-compose could even be helpful

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if we just work with one container, because we can put all

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the run configuration into a file, and we don't have

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to type long docker run commands all the time.

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Now, that was earlier in the course,

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now we have kind of a similar problem with kubectl.

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Instead of writing all these commands manually,

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it would be nice if we could write down our deployment

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and Service Configuration into a file.

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And we could then just apply that file to our cluster

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to then create a new deployment based

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on the configuration in the file, for example.

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And Kubernetes supports that.

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Kubernetes allows us to create

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so called resource definition files,

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which looks something like this.

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And this is just one example of course.

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We can write down configuration options in a yaml file,

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and do that for different kinds

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of objects Kubernetes understands.

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For example, here on this slide for the deployment object

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to define how a deployment should be performed,

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which containers should be launched in this deployment,

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how many instances of our pods we want, and so on.

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And this is an alternative

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to the imperative approach we saw before.

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With the imperative approach,

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we always ran these kubectl commands,

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and we had to execute these individual commands

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to trigger different actions on our Kubernetes cluster.

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This is comparable to using only docker run,

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as I mentioned a second ago.

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Now with the declarative approach, it's different.

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There, we will run another command,

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which we haven't seen thus far, the apply command,

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and will simply point at a yaml file,

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which contains our configuration,

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which we want apply to the cluster.

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And that configuration could contain information

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about a deployment.

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And this conflict file is then used

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to define our desired target state.

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And whenever we apply it,

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Kubernetes will use that target state

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and do whatever it takes to make that the current state.

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And if we then for example, change a configuration file

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and reapply it, Kubernetes will have a look at what changed

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and make the appropriate changes on our running cluster

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and the running application there.

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And therefore data is comparable

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to Docker Compose in a certain sense,

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because now we can use our configuration files

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and we don't have to run a bunch of individual commands.

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So let's see this in action now.